1. Field of the Invention
This invention relates to echo cancellers which anticipate an echo signal which will be superimposed along a signal transmission path, and to subtract the anticipated echo signal from the output. A nonlinear processor or center clipper removes any residual echo that remains in the output signal after subtraction of the anticipated echo, and is arranged to remove residual echo in the output resulting from the far end speaker's signal, and to pass the signal of the near end speaker without distortion. The nonlinear processor of the invention avoids sudden and noticeable variation in the output of the echo canceller by removing residual echo proportionately rather than by operation above a threshold signal level. The nonlinear processor detects the average background noise level and proportionately injects a noise signal in the output to maintain the average level notwithstanding the variation in operation of the nonlinear processor which occurs with the presence or absence of a signal from the near end speaker and the far end speaker, respectively.
2. Prior Art
Numerous echo cancellers are known in the art and are disclosed in prior patents. The general idea of an echo canceller is to determine the transmission response of a transmission path to an impulse input over time, and to calculate an expected echo signal by applying whatever signal which may be received from a remote or "far end" speaker to the characterized impulse response. The expected echo is subtracted from the output of the echo canceller, thus cancelling echo produced by equipment at the near end. Echo can be produced in a transmission path such as a telephone line by impedance mismatches and by coupling between the send (e.g., microphone) and the receive (e.g., speaker) sections of a telephone apparatus. Typically, telephone apparatus includes an analog microphone and acoustic speaker set which is connected via a 2 wire line coupling to a digital signal transmission network through a hybrid analog coupling device and an analog/digital coder/decoder or "CODEC", which may compress and expand the respective signals to reduce the number of bits which must be transmitted over the network digitally. When a party to a conversation produces a signal, any coupling between the send and receive lines remote from that party, for example in the hybrid at the other end of the connection, allows a usually attenuated reflection of the user's signal to be returned over the transmission path, being perceived by the speaker as an echo. Typically, echo from a signal originating at a far end speaker is cancelled by an echo canceller at the near end station, and vice versa.
For purposes of this disclosure, an echo canceller is described with respect to the near end, although both ends preferably are similarly equipped. It is assumed that a two way connection is made between a near end at which the echo canceller is located, and a far end which is to be protected from echo otherwise produced by coupling at the near end. The echo canceller is connected between the transmission lines and the terminal equipment, along a send line (carrying the near end speaker's signal and any residual echo towards the far end) and a receive line (carrying the far end speaker's signal to the near end).
When the far end speaker is the only active speaker, the accuracy of echo cancellation is monitored and used to correct the factors characterizing the expected echo response, because theoretically, when the near end speaker is silent, no signal should be returning on the send-out line coupled to the transmission path. An adaptive control device progressively more accurately characterizes the expected echo, converging on an accurate characterization over a brief period of time.
An initial solution to countering echo in signal transmission was to suppress the echo returned by the near end to the far end by decoupling the send line of the near end station from the transmission line when the far end speaker was active, allowing sufficient time for any echo to subside. However, this also decouples any signal originating at the near end speaker and is noticeable to the parties to the conversation as a coupling and decoupling of the connection. This arrangement is still used with some acoustic applications such as telephone speakerphones.
Similarly, echo cancellers typically include a so-called center clipper, whose main function is to eliminate any residual echo from the send-input (SI) line. When the near end speaker is active, the center clipper is disabled or bypassed, to avoid distortion of the near end speaker's signal. The center clipper is operable when the send-output (SO) exceeds a predetermined threshold (which may be variable as a function of other factors). The center clipper is inoperable when an output higher than the threshold indicates that the near end speaker is active. Unfortunately, this produces the same sort of coupling and decoupling that is noticeable to the parties to the conversation.
When the signal falls below the predetermined threshold that would enable the center clipper, the output signal (SO) goes to zero. When the near end speech (i.e., speech applied to SI) occurs, the center clipper is bypassed, and the error signal with associated circuit noise is gated directly to the send-out (SO) port. As a result, a noise modulated signal is heard by the far end listener. The listener hears background noise (caused for example by room noise at the near end speaker's location or circuit noise in the telephone network) while the near end speaker is speaking, and absolute silence when he or she (the far end person) is speaking. This apparent coupling and decoupling of the connection is annoying, and the annoyance increases as the level of noise applied to the send-in port at the near end increases.
A further related problem with known center clippers is that the send-in noise signal can be of sufficient amplitude to exceed the predetermined threshold at which the center clipper is enabled. When noise peaks exceed the clipping threshold the center clipper is not bypassed. The effect at the far end listener is that crackles are heard rather than the expected sound of white or broadband noise as characteristic of the operative connection.
U.S. Pat. No. 4,577,071 - Johnston et al discloses an echo canceller includes a center clipper which removes peaks from the send-out signal when above predetermined threshold levels. The threshold level is variable as a function of other factors, such as the extent of residual echo. However, the center clipper remains operable as a switching device, producing a coupling and decoupling that is noticeable to the listener.
The present invention eliminates these problems by a particular nonlinear processor coupled to an echo canceller in the same manner as a center clipper. The nonlinear processor includes multiplying means for attenuating the error signal over a continuous range rather than clipping the error signal when the error signal exceeds a threshold. The nonlinear processor also includes a noise injection apparatus sensing the average noise level on the line and inserting a broadband noise signal in the output at a variable amplitude as needed to maintain a constant noise level at the output as the signal from the send-in line (including noise) is being attenuated. The nonlinear processor has the advantages of a center clipper, but avoids the major drawback of known center clippers, namely the noticeable variation in the output signal produced by the center clipper toggling between an operative and an inoperative status.